I was searching for a way to cast a shadow on a plane but I wanted only the shadow to be visible, not the plane. Since I could find a way to do this I finally solved it myself. So maybe this is useful for people who are trying to do the same:
1. Setup your light:
var sunLight1:DirectionalLight = new DirectionalLight(0, -1, 0);
sunLight1.color = 0xFFFFFF;
sunLight1.ambient = 0.4;
sunLight1.castsShadows = true;
scene.addChild(sunLight1);
2. Setup the Shadow
var _shadowMapMethod:SoftShadowMapMethod = new SoftShadowMapMethod(sunLight1, 60);
_shadowMapMethod.range = 12;
_shadowMapMethod.epsilon = .005;
_shadowMapMethod.alpha = 0.6;
3. Setup your plane material
var cMaterial:ColorMaterial = new ColorMaterial(0xFFFFFF);
cMaterial.smooth = true;
cMaterial.specular = 0;
cMaterial.blendMode = BlendMode.MULTIPLY;
cMaterial.ambient = 1;
cMaterial.repeat = false;
cMaterial.lightPicker = new StaticLightPicker([sunLight1]);
cMaterial.shadowMethod = _shadowMapMethod;
4. Create the plane
var _plane:Mesh = new Mesh(new PlaneGeometry(2000, 2000), cMaterial);
_view.scene.addChild(_plane);
_plane.y -= 400;
_plane.z = 500;
Now your objects’ shadows should look like in the attached image.
